1. Field of the Invention
The present invention relates to an acoustic sensor for detecting faint sounds by means of transmission stably with high sensitivity, and an electronic stethoscope device incorporating the same therein. The faint acoustic sounds include a heart sound, a lung sound caused by a polyp in a bronchial tube, a sound of water leakage through cracks in a water main laid under the ground, an acoustic emission (AE), and a sound in a spectrum of seismic waves.
2. Description of Related Art
For convenience in explaining the invention, a range of acoustic frequencies is divided into three sound ranges: a low sound range of 30 to 500 Hz, a middle sound range of 300 Hz to 3 kHz, and a high sound range of 1.5 to 7 kHz. There is no intention to divide the range of acoustic frequencies precisely because the edges of the sound ranges overlapped one another.
In a clinical diagnosis, an airborne-type acoustic sensor, typified by a stethoscope, Is used, almost without exception, to detect sounds within a body under examination, the sounds ranging from a low frequency heart sound to a high frequency lung sound.
The stethoscope, however, causes some -6 dB/oct of roll-off in an airborne sound path from a chest piece to ear chips through rubber tubes. Inevitably, the sensitivity is significantly deteriorated in the high sound range, and this makes it difficult to detect a high frequency body sound, such as a sound from the lungs.
To solve the problem, a peaking method may be adapted, which uses a multi-resonance characteristic appearing when the airborne sound path is regarded as a sound tube. The multi-resonance peaking method results in an improvement of the sensitivity to resonant frequencies, but it is accompanied by significant deterioration of sensitivity in a non-resonant frequency range, resulting in impairment of efforts to the frequency sensitivity characteristics. In addition, the sound tube has a fundamental drawback that resonant frequencies are susceptible to changes in ambient temperature. FIG. 15 shows frequency sensitivity characteristics of a typical stethoscope using a peaking method.
As shown with a dashed-line circle in FIG. 15, multi-point peaking that uses resonance effects of the sound tube compensates for frequency sensitivity characteristics in a mid-frequency sound range.
Alternatively, a highly sensitive acoustic sensor, such as a lung sound sensor, may be used, in which a body sound under examination is detected using a stethoscope designed for exclusive use in a high frequency sound range, picked up by a highly sensitive capacitor microphone, highly amplified and then signal-processed. Yet the system cannot avoid ambient noises: a problem that remains unresolved. The problem is that, even if the frequency sensitivity characteristics of the body sound under examination are compensated by making the best use of signal processing means, including an equalizer and a filter, it is physically impossible to flatten the frequency sensitivity characteristics.
Problems related to conventional stethoscopes have remained unsolved after all, regardless of which of the proposed solutions is adopted. The problems have also been the biggest stumbling block to common use of a database in which wave data of examined body sounds are stored.
Conventional, electronic stethoscopes have such basic structural requirements that a combination of a capacitor microphone and a variable amplifier is incorporated. Such conventional, electronic stethoscope devices are disclosed, for example, in Japanese Patent Laid-open Application Nos. 53-30187, 55-54938, 61-253046, 62-1486531 and 63-135142, Japanese Patent Published Application No. 63-501618, Japanese Patent Laid-open Application Nos. 01-29250 and 02-172449, Japanese Utility-model Laid-open Application No. 03-91310, and M4504A product catalogue of Hewlet Packard company. Those disclosed in the above documents are all configured on the basis of the basic structural requirements.
Another example of related art is disclosed in Japanese Patent Laid-open Application No. 10-258053. This electronic stethoscope device features, instead of the conventional stethoscope, the use of an electret and a pickup, similar to the capacitor microphone, that uses an air gap between the electret and an opposed electrode provided on the body side.
Still another example of related art is disclosed in Japanese Patent Laid-open Application No.10-229984. This electronic stethoscope device features a water bag between an ultrasonic transducer and the body of a patient under examination, so that the focal point of an ultrasonic beam can be adjusted, thereby making an osteoporosis diagnosis of a particular part of the patient with a high degree of precision.
As discussed above, the electronic stethoscope devices provided with the conventional acoustic sensors all use an airborne acoustic sensor, typified by the stethoscope, in which the body sound under examination is transmitted from the body surface to an air layer. The sound waves traveling through the air are picked up by he capacitor microphone, and converted into an electric signal. The electric signal is then amplified and processed.
The following explains the principle of such a stethoscope device mainly used in auscultation examinations. The explanation can also be applied to other techniques for detecting the sounds of leaking, an AE and a seismic sound, merely by reading, as an elastic solid of adequate material, a body of viscous fluid, substantially the same as water, that is subjected to examinations.
From the viewpoint of acoustics, a difference in specific acoustic impedance (hereinbelow, simply referred to as acoustic impedance) between the body and air makes the body sound reflected when it is transmitted from the body surface to the air layer. The reflection loss may be 30 dB or more.
FIG. 14 is a schematic diagram illustrating a sound propagat on system from the body to an acoustic conversion means through a propagation means. In the drawing, if there is a difference in acoustic impedance (Za, Zb, Zc) between the body and the propagation means, or between the propagation means and the acoustic conversion means, the sound is reflected in the boundary at a reflectivity .gamma. given by equation (1) that shows a case where the difference concerned takes place between Z1 and Z2. and .alpha.=Z2/Z1. ##EQU1##
For example, the reflectivity .gamma. is 0.999 between water and air, 0.934 between ceramic and water, or 0.227 between silicon and water. It can be found that the homeomorphous relation between silicon and water has the advantage over the other combinations.
The problem of reflection control also arises with an ultrasonic probe for medical use. This is because the same thing takes place in a heteromorphous relation between liquid and solid in the same way as the relation between liquid and gas in the stethoscope. To solve the problem, a viscous gel has been applied to,the probe before putting the same on the examinee in medical examinations. This solution, however, causes more desensitization and instability for the following reason: The viscous gel is open to the air, and the gel pressure never exceeds the barometric pressure. Therefore, the examiner has no other choice but to apply strong pressure to the examinee.
The intervention of a closed water bag such as one disclosed in Japanese Patent Laid-open Application No. 10-229984 is desirable in terms of acoustic impedance matching because the boundary between homeomorphous liquids can be formed on the body surface. Yet the problem remains unresolved on the other surface that contacts the probe, and it can be said that the position in question is just shifted to the other one. In fact, the intervention of the closed water bag does nothing but adjust the focal length.
In addition, the capacitor microphone has a higher sensitivity; it detects more ambient noises together with the body sound under examination. The higher the sensitivity, the quieter the examination environment should be kept.
That is why water leakage checks on water mains laid under ground are carried out at midnight because of reduced ambient noise.
The diameter of a chest piece of the stethoscope may be made larger to enhance the sensitivity. An excessively large diameter, however, causes remarkable desensitization in the high frequency sound range due to a phase difference caused by non-uniform wave motion in the bore, or uneven propagation lengths, or due to frequency characteristics in the air-borne sound path. In fact, it is set to a proper value within the range of 30 to 50 mm.phi.. Of course, the use of a diaphragm allows the air layer to be sealed effectively, and this improves the sensitivity by several dB. Yet this solution is not essential.